A typical frequency mixer implemented in a radio transceiver comprises a phase-locked loop (PLL) to provide an oscillator signal having a first frequency and one or more frequency dividers dividing the frequency of the oscillator signal output from the PLL to a desired frequency (or frequencies). The output signal of a selected frequency divider is then used for up converting a transmission signal to a desired radio frequency and/or down converting a reception signal from the radio frequency to a baseband.
The performance of a frequency divider is highly dependent on the degree of variation in the level of an input signal. High signal level variation may cause the frequency divider to perform undesired frequency divisions resulting in a distorted oscillator signal in an output of the frequency divider. The degree of the signal level variation may depend on temperature, frequency band, and frequency range supported by the frequency mixer. Signal wires may also cause additional signal variation. For example, jitter is an abrupt and unwanted variation in signal characteristics, such as an interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. Jitter may very well affect the accuracy of rising and falling edges of a digital signal and, thereby, degrade the performance of the frequency divider.
Frequency dividers are typically implemented by digital logic circuits such as counters or flip-flops. Emitter-coupled logic is also known to be used in the frequency dividers. High variation in the input signal of the frequency divider may cause the counter to make a count at an undesired moment, thereby causing an erroneous frequency division. A conventional solution to compensate for the errors caused by signal variation is to deliver the input signal into the frequency divider at a sufficiently high fixed signal level. The problem with this solution is high power consumption and highly variable frequency division performance in cases where the frequency mixer supports a wide frequency range. In the latter case, the high signal level may not be sufficient to fully compensate for a frequency-dependent signal variation. Additionally, the power consumption is a critical factor in devices relying on batteries.